Personally, I dislike it. It's great if you are playing a "gold cheeze" build (Fabulous Treasure + Transmutation Seal, for example) which allows you to level up in there while creating tons of popcorn on the main floor, but more or less useless if you aren't. Under normal circumstances, why would I ever want to waste gold on spawning an equal level monster? Perhaps if I found both WONAFYT and a Balanced Dagger this could be an interesting prospect, but I don't think it's worth it outside of these special circumstances. I 90% of the time just see it as wasted potential.

My thoughts are:1. It's too expensive to spawn one equal level monster for 5g in most runs2. +1 level enemy would be more useful

I just saw it for the first time. I utilized it for some extra level 8's to get me closer to a level when it was late game and I had gold to spare. But it seems to be highly dependent on class and how the run is going. That may be true of a lot of sub dungeons.

Agreed, it's too expensive for what it offers. Assassins can make pretty good use of it to eke out a bit more XP (spawn a level 8, level up and swift hand it, spawn a level 9, level up and swift hands it; 17 XP for 10 GP isn't a bad deal) but for everyone else it's hard to get any utility whatsoever. At the prices it's charging, it should just be giving away free XP (an effect that would be comparable to the Grove of Knowledge).

It's situational like everything else. I found myself saving it up for levels 6+ so I get more xp. It's bonus xp if you run out of mobs early. Helpful sometimes. Bypasses Labyrinth mob counter too, I think.